Alloy steel is a combination of iron and up to 50% by weight of alloying elements such as nickel, chromium, molybdenum, manganese, vanadium, silicon, and boron. Alloy steels have greater strength, hardness, hot hardness, wear resistance, hardenability, and/or toughness compared to carbon steel. Alloy steels are further characterized by the amount of alloy added, with low alloy steels typically less than 2% to 4% by weight alloying, and high alloy steels having greater than 4% by weight alloying.
Examples of alloy steels used in commercial applications include NiCrMoV and CrMoV alloy steels. For example, NiCrMoV and CrMoV alloy steels are often included in shafts, flanges, wheels, and disks included in a gas turbine. Heat treatment of these and other alloy steels is typically required to improve the strength, hardness, wear resistance, and/or toughness characteristics of the alloy steels compared to carbon steel.
Gas turbine components typically operate in an environment of more than 600° F. for extended periods of time. Prolonged exposure of alloy steels to high operating temperatures results in thermal embrittlement of the alloy steel, particularly at or near the surface of the alloy steel. For example, thermal embrittlement is commonly experienced around bores and/or bolt holes after prolonged operations at high temperatures. The thermal embrittlement produces slight micro-structural changes in the alloy steel that reduce the fracture toughness of the alloy steel and limit the service life of the alloy steel components. The service life of alloy steel components may be determined according to various metrics, such as the amount of time that the alloy steel component is exposed to high temperatures or a decrease in the fracture toughness and/or fracture appearance transition temperature (FATT) of the alloy steel component. Any of these parameters, or others, may be used to predict the onset of thermal embrittlement which may result in excessive crack formation and/or propagation.
Alloy steel components that have experienced thermal embrittlement due to prolonged exposure to high temperature environments may be annealed using heat treatment methods known in the art. For example, the alloy steel components may be disassembled or removed and placed in a suitable chamber to heat the alloy steel components. However, the size of the alloy steel components often requires a correspondingly large chamber. In addition, removal and disassembly of the alloy steel components is time consuming, expensive, and results in extended maintenance periods during which the commercial equipment is not available for operation.
Therefore the need exists for an improved system and method for annealing alloy steel components. Ideally, the improved system and method may anneal localized areas of the alloy steel components to reduce the time and cost associated with the annealing process, thereby reducing the amount of time that the commercial equipment is inoperable.